We have continued our studies of differentiation and development in the nervous system by analysis of poly(A)containing messenger RNA population in hybridization reactions with complementary DNAs made from them. We have found, by recycling hybridizations of cDNAs from undifferentiated and highly differentiated mouse neuroblastoma cells, that there are mRNA sequences peculiar to both differentiation states. By kinetic analysis of hybridization curves, it appeared that there were approximately 3-4 high abundance and 320 low abundance sequences specific to differentiated cells. There appeared to be 3 high abundance and 250 low abundance messenger sequences specific to the undifferentiated cells. We have also begun to analyze for differences among the non-histone chromatin proteins from cells in both differentiation states. In addition, changes in the sequence complexity of chicken liver RNA after treatment of the animals with vitellogenin were measured.